THE TRANSCRIPTIONAL TRANSACTIVATOR OF SIMIAN FOAMY VIRUS-1 BINDS TO ADNA TARGET ELEMENT IN THE VIRAL INTERNAL PROMOTER

The transcriptional transactivator (Tas) of simian foamy virus type 1 strongly augments gene expression directed by both the promoter in the viral Long terminal repeat and the newly discovered internal promoter Located within the env gene, A region of 121 bp, located immediately 5' to the TATA box in the internal promoter, is required for transacti vation by Tas. The present study aimed to identify the precise Tas-res ponsive target(s) in this region and to determine the role of Tas in t ranscriptional regulation, By analysis of both clustered-site mutation s and hybrid promoters in transient expression assays in murine and si mian cells, two separate sequence elements within this 121-bp region w ere shown to be Tas-dependent transcriptional enhancers. These targets , each <30 bp in length and displaying no apparant sequence homology o ne to the other, are designated the promoter-proximal and promoter-dis tal elements, By means of the gel electrophoresis mobility-shift assay , using purified glutathione S-transferase-Tas fusion protein expresse d in Escherichia coli, the target proximal to the TATA box exhibited s trong binding to glutathione S-transferase-Tas, whereas the distal ele ment appears not to bind, In addition, footprint analysis revealed tha t 26 bp in the promoter proximal element was protected by glutathione S-transferase-Tas from DNase I. We propose a model for transactivation of the simian foamy virus type 1 internal promoter in which Tas inter acts directly with the proximal target element positioned immediately 5' to the TATA box, In this model, Tas attached to this element is pre sumed to interact with a component(s) of the cellular RNA polymerase i i initiation complex and thereby enhance transcription directed by the viral internal promoter.